The present invention is directed generally to horn antennas, and more specifically to a new class of hybrid-mode horn antennas having selective gain.
Maximum directivity from a horn antenna is obtained by uniform amplitude and phase distribution over the horn aperture. Such horns are denoted as “hard” horns. They can support the transverse electromagnetic (TEM) mode, and apply to linear as well as circular polarization. They are characterized with hard boundary impedances:Zz=−Ez/Hx=0 and Zx=Ex/Hz=∞,  (1)                or soft boundary impedances:Zz=Ez/Hx=∞ and Zx=Ex/HZ=0,  (2)        meeting the balanced hybrid condition:ZzZx=η02,  (3)        where η0 is the free space wave impedance and the coordinates z and x are defined as longitudinal with and transverse to the direction of the wave, respectively.        
Hard horns can be used in the cluster feed for multibeam reflector antennas to reduce spillover loss across the reflector edge. Such horns may also be useful in single feed reflector antennas with size limitation, and in quasi-optical amplifier arrays.
Two different hard horns which meet these conditions are one having longitudinal conducting strips on a dielectric wall lining, and the other having longitudinal corrugations filled with dielectric material. These horns work for various aperture sizes, and have increasing aperture efficiency for increasing size as the power in the wall area relative to the total power decreases. Dual mode and multimode horns like the Box horn can also provide high aperture efficiency, but they have a relatively narrow bandwidth, in particular for circular polarization. Higher than 100% aperture efficiency relative to the physical aperture may be achieved for endfire horns. However, these endfire horns also have a small intrinsic bandwidth and may be less mechanically robust. Linearly polarized horn antennas may exist with high aperture efficiency at the design frequency, large bandwidth and low cross-polarization. However, these as well as the other non hybrid-mode horns only work for limited aperture size, typically under 1.5 to 2λ.